1. Field of the Invention
The present invention relate to an optical analysis method for inhomogeneous turbid media.
2. Related Background Art
Probing, measurement, analysis and the like in a medium (substance) using light have been studied. When a medium is irradiated with light, light absorption and scattering occur in the medium. The light absorption and scattering change depending on the physical quantity of each medium constituent material. For this reason, when transmitted light or reflected light from the medium is measured, the physical quantity can be determined on the basis of the measurement value. For example, the light absorption coefficient of a component contained in the medium can be determined on the basis of the measurement value.
Especially, when the medium exhibits not only absorption but also light scattering, the physical quantity is determined in consideration of the scattering coefficient. In a simple case, fundamental expressions are known. Japanese Patent Laid-Open No. 2-234048 discloses a measurement example for a case using a scattering coefficient. In this prior art, the scattering and absorption are analyzed using a time variable on the basis of the transient waveform of a measured light intensity.
Japanese Patent Laid-Open No. 4-297854 is known as a physical quantity analysis example using an exponential function similar to that of the present invention. This prior art discloses a method of correcting an optical diffusion effect in a medium. An exponential function is used as an absorbance correction function.
In a homogeneous medium as well, the effective optical path length increases due to optical diffusion. When the effective optical path length increases, the light intensity attenuates. Hence, in the physical quantity analysis of this prior art, an exponential function having a constant whose physical contents are undefined is empirically introduced in correcting the influence of a change in absorbance, thereby correcting the apparent light intensity. The same phenomenon as described above also occurs in an inhomogeneous medium, though the prior art does not cover any inhomogeneous medium. As described above, in physical quantity analysis for a medium, many methods that take the absorption coefficient and scattering coefficient into consideration are known.
However, conventionally, when the medium exhibits light scattering and has an inhomogeneous structure, i.e., when the medium is not homogeneous but is buried in a translucent white medium, no physical quantity analysis method therefore is known. Typical inhomogeneous/scattering substances are natural substances and living organisms. The inhomogeneous medium can be approximately regarded as a homogeneous medium and analyzed, as a matter of course. however, when parts have different optical characteristics, and interscattering between the different parts occurs, the degree of light attenuation in the inhomogeneous medium cannot be described by the conventional theory. In addition, since a different refractive index results in a different light speed, an analysis method based on an assumption that light propagates at a predetermined speed cannot be used. Microscopic analysis such as the Monte Carlo method cannot be solved without any precise assumption of inhomogeneous structure distribution and therefore cannot be generally used. As described above, with the conventional method, accurate physical quantity analysis cannot be performed.
It is an object of the present invention to provide an optical analysis method for an inhomogeneous medium, in which, for a medium in an inhomogeneous, polyphase, or mixed state, the components of constituent materials of the medium are separated, and relations to the light intensity are defined, thereby allowing accurate physical quantity analysis for the inhomogeneous medium.
According to the present invention, in an optical analysis method for an inhomogeneous medium, in which light is made incident into an inhomogeneous medium, the intensity of light that emerges from the inhomogeneous medium is detected, the detected light intensity is substituted into a predetermined function, and the physical quantity of the inhomogeneous medium is determined on the basis of the function, solutions of simultaneous differential equations that describe a condition in which light according to light scattering between a plurality of different parts mixes are expressed by a descriptive function. The function is a function that defines the relationship between the light intensity and the linear sum of exponential functions of the penetration depth of the light, which depend on the physical quantity of the inhomogeneous medium and uses e as a base. In this case, the physical quantity of the inhomogeneous medium can be accurately analyzed.
The physical quantity is defined by the absorption coefficient of the inhomogeneous medium, the scattering coefficient of the inhomogeneous medium, or the concentration of a predetermined component in the inhomogeneous medium.
As a characteristic feature, letting I be the intensity, t be the penetration depth, and xcex1 and xcex2 be unknowns depending on the physical quantity, the function is given by I=(exe2x88x92xcex1t+exe2x88x92xcex2t)/2.
As a basic characteristic feature, the light that emerges due to reflection is light reflected by the inhomogeneous medium, and the light amount (intensity) of the reflected light is represented by the linear sum of (xcex1+xcex2)/xcex1xcex2 and 1/(xcex1+xcex2).